Emission control devices, such as diesel particulate filters (DPF), may reduce the amount of particulate matter emissions (such as, soot) from a diesel engine by trapping the particles. Such devices may be regenerated during engine operation to decrease the amount of trapped particulate matter (for example, by burning) and maintain the collection capacity of the device. To meet stringent federal government emissions standards, regeneration operations and DPF functionality may be tightly controlled and regularly assessed.
One example approach for controlling diesel particulate filters is illustrated by Stewart et al. in U.S. Pat. No. 7,155,334. Therein, an engine controller controls filter regeneration based on inputs received from sensors, such as particulate matter sensors and/or carbon dioxide sensors, positioned upstream and downstream of the filter.
However, the inventor herein has recognized issues with such an approach. As one example, the use of resistive sensing-based particulate matter (PM) sensors reduces the ability of an engine control system to differentiate between a degraded filter and a degraded regeneration operation. As such, commonly used PM sensors may be configured to detect the presence of PMs electrically, based on a change in resistance or capacitance across an electrical circuit. Such sensors may have a “dead-band” during which PMs may have to accumulate before the sensor is able to respond. This additional time required to detect PMs may reduce the electrical sensor's sensitivity to a degraded DPF. Similarly, relatively small differences in resistance may reduce the ability to distinguish between a degraded DPF and a marginal DPF. Thus, in one instance, the system may not be able to identify DPF degradation.
As another example, the use of input from CO2 sensors that sense exhaust CO2 levels may also reduce the system's ability to accurately estimate the soot load on the filter due to an indirect correlation between filter soot levels and exhaust CO2 levels. Since the exhaust CO2 level is more representative of combustion conditions, a soot load may be inferred but not accurately determined.
Thus, in one example, some of the above issues may be addressed by a method of operating an engine exhaust system including a particulate filter comprising, controlling filter regeneration based on a CO2 signature of oxidized, post-filter exhaust particulate matters (PMs). The CO2 signature may include a CO2 level of oxidized PMs estimated by a CO2 sensor positioned downstream of the filter
In one example, a diesel engine exhaust system may be configured with a filter substrate and a CO2 sensor positioned downstream of a DPF. During filter regeneration, an engine controller may heat the substrate and oxidize post-filter exhaust particulate matters (that is, exhaust soot) on the heated substrate using oxygen present in the exhaust gas. The substrate may be heated by the flow of hot exhaust gas, as used during filter regeneration, through the substrate. The CO2 generated during regeneration, from oxidation of exhaust soot on the substrate, may be estimated by the downstream CO2 sensor to determine a regeneration CO2 signature of oxidized, post-filter exhaust particulate matters (PMs). The regeneration CO2 signature may at least include a CO2 level of the oxidized PMs. Since the generated CO2 is largely dependent on the quantity of exhaust soot oxidized on the heated substrate, a direct correlation may be made between the estimated exhaust CO2 level and an exhaust soot level. In other words, the CO2 sensor may be used as a PM sensor. The CO2 level may be monitored, over at least a duration of the regeneration, to perform filter diagnostics and/or assess the efficiency of the regeneration operation. Specifically, the controller may indicate filter degradation based on the CO2 signature. The controller may then adjust engine and filter operations based on the CO2 signature. During other engine running conditions, the CO2 sensor may be used to sense an exhaust CO2 level unrelated to post-filter exhaust PMs.
For example, based on regeneration conditions, such as an estimated soot load, burn rate, exhaust temperature, exhaust flow rate, etc., an engine controller may determine an expected regeneration CO2 level, signature, and/or profile. A CO2 level of post-filter oxidized exhaust PMs may then be estimated in real-time during regeneration, for example, at fixed intervals since the initiation of the regeneration operation, and compared to the expected values. As such, low to substantially no exhaust PMs may be expected in the post-filter exhaust. Herein, by comparing the output of the CO2 sensor to CO2 levels expected based on the engine's operating conditions and regeneration conditions, exhaust PMs may be identified in the post-filter exhaust and may be used to infer filter degradation. In one example, based on the comparison, the controller may determine whether the filter is degraded and further whether the regeneration operation is degraded. If there is no degradation, the sensor output may be used to infer whether regeneration has been completed or not and to adjust engine operations accordingly. For example, the controller may identify a degraded filter when the estimated regeneration CO2 level (that is, sensor output) is higher than the expected regeneration CO2 level. Similarly, the controller may identify a degraded regeneration operation when the estimated regeneration CO2 level is below a threshold. The controller may adjust regeneration conditions (such as the burn rate, flow rate, etc.) in real-time for the same operation, or for a subsequent operation responsive to the indication of filter and/or regeneration degradation. For example, if the filter is degraded (e.g., cracked) and PMs are being detected in the post-filter exhaust, the regeneration conditions may be restricted to lower temperatures, lower durations, lower soot load thresholds, etc., so as to reduce the risk of further PM slip into the exhaust emissions. In another example, if the regeneration is degraded (e.g., not enough of the stored soot is being burned effectively), the regeneration conditions may be reconfigured to higher temperatures, higher burn rates, longer durations, higher soot load thresholds, etc., so as to increase the amount of stored soot that is burned off. In addition to controlling filter regeneration, the engine controller may adjust alternate engine operations based on the CO2 signature.
It will be appreciated that while the depicted example illustrates application of the CO2 sensor in a diesel engine exhaust system, this is not meant to be limiting, and the same CO2 sensor may be similarly applied in alternate engine exhaust systems, such as to diagnose a gasoline particulate filter in a gasoline engine exhaust system.
In this way, the presence of soot in engine exhaust may be detected by oxidizing the soot to generate CO2, and by using downstream CO2 sensors to provide a more direct and more precise estimate of exhaust soot levels, in addition to their use in estimating exhaust CO2 levels. By enabling an accurate, real-time estimate of exhaust soot levels, filter regeneration may be better controlled. Additionally, the higher sensitivity of the CO2 gas sensors may reduce the “dead-band” effect of resistive sensors and provide higher resolution between signals. This higher resolution may improve the ability to identify a degraded particulate filter, and further to distinguish between a degraded filter and reduced filter functionality. Similarly, the higher resolution may improve the ability to distinguish between a degraded filter and a degraded regeneration operation. By improving regeneration and filter diagnostics, the quality of exhaust emissions maybe improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.